FIG. 1 schematically illustrates a procedure of forming circuit patterns on upper and lower sides of an insulation layer and electrically connecting the circuit patterns in a related art printed circuit board (PCB).
Referring to FIG. 1, a raw material (flexible copper clad laminate, FCCL) in which conductive layers are disposed on both sides of an insulation layer is prepared first in the related art PCB. It is illustrated that a polyimide film is used as the insulation layer, and a copper film is used as the conductive layer.
Subsequently, blanket etching is performed. Since a copper foil of the FCCL has a fixed thickness and a thickness of the FCCL is increased to or more when a through hole is plated, the FCCL becomes too thick in the case of intending to form micro-patterns, thus making it difficult to implement a precision circuit through etching. For this reason, before processing of through holes, blanket etching is performed to reduce the thickness of the FCCL.
Thereafter, the conductive layer and the insulation layer are processed to form through holes passing therethrough. Then, a pre-coating process is performed by exposing the conductive layer and the insulation layer, where the through holes are formed, to a conductive aqueous solution and thus forming a conductive film (shadow process).
Afterwards, a pre-copper electroplating process is performed by forming an electroless copper plating layer on the conductive layer and the insulation layer where the conductive film is formed, and an inner wall of the through hole is coated with a conductive copper thin film using palladium (Pd) catalytic reaction. The inner wall of the through hole is fully coated with conductive copper through electrolysis of copper.
Next, a photoresist layer is laminated, and then exposure, development, etching and delamination are performed to form a circuit with desired patterns, thereby completing a final circuit.
In this way, according to the related art PCB, a process of electrically connecting the circuit patterns formed on both sides of the insulation layer through the through holes is too complicated, leading to a decrease in productivity and an increase in failure rate.